The human gastrointestinal (Gl) tract is colonized by an abundant and diverse microbiota. Numerous factors (e.g., antibiotic treatment, diet, etc.) may alter the normal and beneficial bacterial flora of the gut, which may lead to various chronic and degenerative diseases that are particularly problematic in young children and the elderly. One of the approaches used to alleviate those disorders is the ingestion of probiotic bacteria; i.e., various nonpathogenic bacteria (e.g., lactobacilli and bifidobacteria) that may improve health when ingested in sufficient quantities. However, a potentially useful probiotic intervention that has not received much attention is to use bacteriophages to target "problem" bacterial species in the human Gl tract. Bacteriophages (or phages) are viruses that infect bacteria. They are the most abundant microorganisms on earth, and they play a key role in maintaining balanced bacterial populations in numerous ecosystems. Lytic phages have remarkable antibacterial activity; however, in contrast to antibiotics, they are very specific; i.e., they only lyse strains of their targeted bacterial species. The studies proposed in this application address our hypothesis that, because of their high specificity, bacteriophages may be useful in favorably manipulating the microflora of the mammalian intestinal tract (by targeting specific "problem" bacterial pathogens in the Gl tract), and that phage-based probiotics will have no deleterious effect on the beneficial microflora of the gut. We will begin to evaluate our hypothesis by using L. monocytogens and a L monocytogenes-specific phage preparation (LMP-102) as a model system. We propose that making a L monocytogenes-specific phage preparation part of a routine diet may significantly reduce the risk of ingested L. monocytogenes colonizing the Gl tract and proliferating to sufficient levels to cross the intestinal mucosa and disseminate via the mesenteric lymph nodes and bloodstream. The following three specific aims will be addressed: SA #1: Develop genetic tools to identify and monitor individual monophage components of L. monocytogenes-spectflc phage preparation (LMP-102) in vitro and in vivo; SA #2: Determine the persistence of LMP-102 monophages in the Gl tract of mice (in the presence and absence of the targeted L. monocytogenes strain) and determine their effect on L. monocytogenes colonization in experimentally-infected mice; SA #3: Compare the in vivo effects of LMP-102 and ampicillin on the intestinal microflora of healthy mice and mice experimentally-infected with L monocytogenes. The studies in this application are designed to yield critical preliminary data concerning bacteriophage-bacterial interactions in the Gl tract, and they may lead to the development of novel probiotic preparations for the prophylaxis and/or treatment of illnesses caused by various bacterial pathogens that have an oral portal of entry and require short- or long-term colonization in the Gl tract in order to cause disease. [unreadable] [unreadable] [unreadable]